Electrostatic printing process wherein development is achieved by sequenctial application of carrier liquid and developer particles



United States Patent ELECTROSTATIC PRINTING PROCESS WHEREIN DEVELOPMENTIS ACHIEVED BY SEQUENTIAL APPLICATION OF CARRIER LIQUID AND DE- VELOPERPARTICLES Paul E. Wright, Haddonfield, N.J., assignor to RadioCorporation of America, a corporation of Delaware Filed Sept. 29, 1961,Ser. No. 141,711 2 Claims. (Cl. 96-12) This invention relates generallyto electrostatic printing and more specifically to improved methods andmeans for developing electrostatic images.

In the art of electrostatic printing, electrostatic images are producedon an insulating surface and are then developed into visible images withelectroscopic developer particles. The electrostatic images may beproduced by direct charge deposition as by selectively energizing pinelectrodes to deposit charges on an insulating plastic surface in a dotpattern. Electrostatic images may also be producedelectrophotographically as by subjecting a photoconductive insulatingsurface to substantially uniform discharge from a corona generatingdevice and then exposing the surface to a light image to discharge theilluminated areas thereon to thereby produce the electrostatic image. Ineither case, the electrostatic image can be developed into a visibleimage by various methods such as cascade development, magnetic brushdevelopment, or powder cloud development, all well known in the art. Inrecent times, methods of developing electrostatic images have beendevised wherein a dispersion of electroscopic particles in an insulatingliquid is employed and the electrostatic image developed by cataphoreticdeposition of particles from the liquid. The developer dispersions maybe applied to electrostatic images on a surface by flowing thedispersion thereover, by immersing the surface in the dispersion or bypassing a roller coated with dispersion over the surface.

While developing methods employing liquid dispersions provide manyadvantages over those achieved with methods employing dry powders, thereare, often times, attendant disadvantages. For example, theaforementioned flowing and immersion techniques frequently require moretime for development than methods employing dry powders. Liquiddevelopment using rollers can introduce a problem with respect to offsetprinting of ghost images. While liquid developing techniques gener allyproduce higher resolution images, difficulties can be experienced infilling in large, solid-colored image areas.

Some liquid developer dispersions possess a self-fixing property suchthat, when an electrostatic image is developed therewith, the developedimage need not be heated or otherwise treated to render the imagepermanent. Such dispersions include a binder material dissolved in ahighly volatile carrier liquid which, while it is a solvent for thebinder, is not a solvent for the developer particles dispersed therein.Self-fixing liquid developers are highly desirable but may introducevexatious problems. Due to the volatility of the carrier liquid,evaporation thereof rapidly changes the proportions of the developerparticles and the binder material in the carrier liquid. Such a changein proportions can result in the production of prints varying incontrast and can even completely negate the effectiveness of liquiddevelopment. Rapid evaporation of carrier liquid in developer apparatussuch as, for example, roller developing apparatus presents anotherproblem in that water condensation results on the rollers andcontaminates the developer composition. Any apparatus employing a liquiddeveloper containing a binder will require frequent cleaning to preventbinder material from gumrning up in the apparatus.

Accordingly, it is a general object of this invention to provideimproved methods, devices, and apparatus for applying liquid developercomposition to electrostatic images.

It is another object of this invention to provide improved methods,devices and apparatus for developing electrostatic images at highspeeds.

Yet another object is to provide improved methods, devices and apparatusfor developing electrostatic images with improved fill-in of large,solid-colored image areas.

It is also an object of this invention to provide improved methods,devices and apparatus for developing electrostatic images with a liquiddeveloper composition without requiring either a binder material or ahighly volatile carrier liquid for producing self-fixing visible images.

These and other objects and advantages are achieved in accordance withthe methods of this invention by contacting across an electrostaticimage an absorbent pad having dispersed therein a quantity ofelectroscopic developer particles. At least that portion of theabsorbent pad which is cont acted to the electrostatic image is wettedwith an insulating carrier liquid. This may be accomplished in any oneof several ways. For example, an absorbent pad, containing a drydipersion of developer particles, can be contacted across anelectrostatic image to which there has been previously applied a thinlayer of insulating liquid. An absorbent pad,- containing a drydispersion of developer particles, can be dipped in an insulating liquidand then contacted across an electrostatic image. A quantity of liquiddeveloper composition comprising developer particles di-psersed in aninsulating liquid can be absorbed into the pad and thereafter contactedacross an electrostatic image. In any oase, as the pad is moved across asurface bearing an electrostatic charge image, the electroscopicparticles are electrostatically attracted from the pad and deposit onthe surface in configuration with the charge image, Images can bedeveloped on a surface substantially at the maximum speed that the padcan be manually wiped over the surface. I

Development of electrostatic images with absorbent pads, as aforesaid,also provides novel methods of producing visible images in a pluralityof colors by successively developing different electrostatic images on asingle insulating or photo-conductive surface with different coloreddeveloper particles. When a volatile carrier liquid is used in a methodof this invention, a developed image can be produced which isself-fixing and has more resistance to smearing than does commercialnewsprint.

This invention also includes a disposable article of manufacture. Suchan article comprises an absorbent pad, such as for example, absorbentcotton having a quantity of developer particles dispersed therein or onewhich has been soaked with, but not saturated with, a developercomposition comprising electroscopic developer particles dispersed in aninsulating liquid. Such an article, containing a liquid developercomposition, may be supplied to an ultimate user in a sealed container.When the development of images with the pad results in depletion of thedispersion contained therein, it can be discarded. Means can also beprovided for replenishing the dispersion contained in the pad andthereby extend the useful life thereof.

Improved apparatus for use in electrostatic printing equipment is alsodisclosed herein. Such apparatus includes an absorbent pad which iscontacted across an electrostatic image bearing surface. Means areprovided for feeding developer composition to the pad while it is inoperation to maintain a sufficient amount of developer composition inthe pad. With such apparatus, depletion of developer compositionpresents no problem and operating life is limited only by normal Wear.

Additional objects and advantages will be evident from the followingdetailed description which is illustrated in the accompanying drawingswherein:

FIGURE 1 is a perspective view of one embodiment of a developer padencased in a sealed container in accordance with this invention;

FIGURE 2 is a perspective view illustrating a method of using thedeveloper pad of FIG. 1 to develop an electrostatic image;

FIGURE 3 is a perspective view, partly in cross section, of anotherembodiment of a developer device in accordance with this invention;

FIGURE 4 is a view in elevation and partly in cross section of a thirdembodiment of a developer device in accordance with this invention inwhich developer fluid is added to a developer pad continuously duringuse.

Similar reference characters are applied to similar elements throughoutthe drawings.

As shown in FIGURE 1, a developer device, in accordance with oneembodiment of this invention, comprises a fabric pad of absorbentmaterial containing a dispersion of electroscopic particles dispersed inan insulating liquid. The fabric pad 11 preferably comprises a cottonmaterial such as, for example, a cotton wicking material. Almost equallyeffective are other absorbent materials such as felt padding and fiberglass padding materials. As illustrated, the pad is enclosed in anairtight, or nearly so, container 12 in order to prevent evaporation ofthe insulating liquid before the pad is used for developingelectrostatic images.

In fabricating a suitable developer device, a liquid de velopercomposition is first formulated. Such a composition may include, forexample, an insulating carrier liquid consisting of about equal parts oftrichlorotrifiuoroethane and a dimethyl polysiloxane having a viscosityof about 3 centistokes or less. Many well-known developer powders may bedispersed in the carrier liquid to provide the liquid developercomposition. For example, a suitable black powder may comprise 200 partsby weight of thermoplastic resin such as, for example, Piccolastic Resin4358A (Pennsylvania Industrial Chemical Corp., Clairton, Pa.); 12 partsby weight of carbon black; 12 parts by weight of Nigrosine SSB (ColorIndex No. 50415) and 8 parts by weight of Isosol Black (Color IndexSolvent Black 13). The developer powder is prepared by melting the resinand mixing in the other materials. When a uniform mix is obtained, it iscooled, ground to a fine powder and classified as to patricle size. Aconvenient particle size is one obtained by screening through a 200 meshor finer screen to provide a maximum particle diameter about equal to orless than 74 microns. Up to about 6 parts by weight of developer powderare stirred into about 100 par-ts by weight of the carrier liquid toprovide a liquid developer composition.

A developer pad about 3: to 4 inches long and to /2 inch wide, which hasbeen dipped once in a developer composition, will provide a developerdevice capable of developing 50 or more electrostatic images on 8 /2 by11 inch surfaces.

FIGURE 2 illustrates a method of manually developing an electrostaticimage on an insulating surface 14. A developer pad 11 containingdeveloper composition is held in an ordinary spring clip 16 to provideconvenient means for handling the pad 11. The pad is then contacted toand swept across the insulating surface 14. As it is swept across thesurface 14 developer powder particles are electrostatically exrtactedfrom the pad 11 and from the carrier liquid in the pad. The extractedparticles are electrostatically held in the charged areas on the surface14 to form thereon a visible powder image 18. Images can be developed inthis manner as fast as the pad can be swept across the surface. When anelectrostatic image is developed in this manner, using the developercomposition described heretofore, the carrier liquid extracted from thecomposition evaporates almost immediately from the insulating surface 14providing a fixed black image 18 on the surface 14. This image 18 is atleast as resistant to smearing as is ordinary newsprint.

By using a plurality of pads such as the pad 11 of FIGURE 1, thepreparation of color reproductions is made possible. Each pad is used todevelop a different electrostatic image in a different color. Forexample, an absorbent pad is prepared for developing an electrostaticimage into a black image using the developer composition describedheretofore. Developer compositions for producing other colors can beprepared by dispersing color pigments or combinations thereof in acarrier liquid comprising about equal parts by volume of dimethylpolysiloxane and trichlorotrifluoroethane. It is preferred that thecompositions contain about 0.005 to 6% by weight of color pigment andthat the pigment or pigments be substantially insoluble in the carrierliquid.

Many color pigments can be incorporated into developer compositions tobe absorbed into the pad 1 1 of FIGURE 1. The following comprises apartial list of those which have been found to be suitable:

(1) Cyan blue toner GT (described in U. S. Patent 2,486,-

351 to R. H. Wiswall, Jr.)

(2) Benzidine Yellow (3) Brilliant Oil Blue BMA (Color Index No. 61555)(4) Sudan III Red (Color Index No. 26100) (5) Oil Yellow 2G (Color IndexNo. 11020) (6) Hansa Yellow 2G (Color Index No. 11680) (7) PyrazalonePigment Red 39 (Color Index No. 21080) Color reproductions can be veryconveniently produced using a plurality of absorbent pads eachcontaining a different color developer dispersion. Anelectrophotographic method of color reproduction can be practiced in thefollowing manner. A dye-sensitized electrophotographic sheet is chargedto a substantially uniform surface potential. Dye-sensitizedelectrophotographic sheets and the charging thereof are described inElectrofax, Direct Electrophotographic Printing on Paper by C. J. Youngand H. G. Greig RCA Review, December 1954, vol. XV, No. 4. The chargedsheet is exposed through filters to an original color transparency, thefilters being selected to mask that light which corresponds to the onecolor component which it is desired to reproduce. A latent electrostaticimage is thus produced which is representative of the one colorcomponent of the original. An absorbent developer pad containing anappropriate colored developer composition is then used to develop theelectrostatic image. Repeating these steps, each time with difierentcolor filters and different color developer compositions results in thesuperimposition of one color over another on the electrophotographicsheet to provide a substantially faithful reproduction of the originalcolor image.

Because each color, as it is reproduced on the electrophotographicsheet, is self-fixing, no smearing of one color results when thedeveloper pad is wiped across that color during a subsequent step.Colors reproduced in this manner are, in fact, substantially moreresistant to smearing than is ordinary newsprint. Also, because no heatis required to fix a color to an electrophotographic sheet such aspaper, there is no danger of producing dimensional changes in a papersheet during processing as would be the case if each color had to beheat fixed thereto. Because of this fact, registration of superimposedcolors is greatly simplified.

The developer pad 11 can also be provided with a dry dispersion ofdeveloper particles. This can be accomplished, for example, bydispersing developer particles such as carbon black or the colorpigments mentioned heretofore in a volatile non-solvent liquid such astrichlorotrifiuoroethane. The pad 11 is soaked in or saturated with thisdispersion and the liquid allowed to evaporate therefrom. A drydispersion of developer particles in the absorbent pad 11 results. Anelectrostatic image is developed with this pad 11 by merely wetting atleast that portion of it which is contacted across the image. Forexample, the developer pad containing the dry dispersion can be dippedin a tray containing an insulating liquid such as dimethyl polysiloxaneand thereafter wiped across an electrostatic image to develop it.

Another novel method of color reproduction can be realized employingdeveloper pads each containing a dry dispersion of a different coloredpigment. A thin film of a slow evaporating carrier liquid such asdimethyl polysiloxane is applied to an electrophotographic sheet bywiping thereacross a pad containing such liquid. Electrostatic imagescorresponding to color components of an original color image arethereafter reproduced and developed as described heretofore. During eachdevelopment step, a pad containing a different colored pigment isemployed, the siloxane liquid on the sheet acts as the carrier for eachpigment and remains on the sheet for a time sufficient to produce allcolors in the reproduction. An unusual feature of this method of colorreproduction resides in the fact that, although the sheet is wetted withcarrier liquid throughout the time required to produce all colors, nosmearing results when each successive color is developed on the sheet.Once a color reproduction has been made, the sheet can be set aside todry or, if desired, drying can be accelerated by heating.

Another embodiment of this invention, illustrated in FIGURE 3, comprisesa rectangular fabric pad 11 carried by a holder 20, the handle portionof which indludes a cavity 22 filled with liquid developer composition.To prevent evaportion of liquid from the composition during shipment andwhen the device is not in use, a removable rectangular cap 24 isprovided which fits over the fabric pad 12 and the lower end of theholder 20. The holder 20 is preferably made of a flexible material suchas polyethylene so that developer composition extracted from the pad 12can be replaced by applying 'sligh-t pressure to the holder 20 adjacentthe cavity 22 therein.

Electrostatic printers have been devised for recording on continuouswebs at high speeds. One such printer records on 8 /2 inch wideelectrophotographic paper from a cathode ray tube at rates of more than10,000 characters per second and paper speeds of 18 inches or more persecond. More details with respect to such a machine are included in anarticle entitled A Thin-Window Cathode- Ray Tube for High Speed Printingwith Electrofax by R. G. Olden, RCA Review, September 1957, vol. XVIII,No. 3. It can be readily understood that development of electrostaticimages at such high speeds introduces problems with respect to properformation of visible images and with respect to rapid depletion ofdeveloper material.

Many problems in high speed development of electrostatic images areovercome by providing a developer apparatus such as that illustrated inFIGURE 4. In this apparatus, a continuous web 30 of Electrofax paper isdrawn over a roller 32 and in contact with a developer pad 11. Thedeveloper pad 11 is supported in a substantially rectangular housing 34which contains a reservoir 36 of liquid developer composition. Developercompositions are fed into one end of the reservoir 36 through a conduit38. A centrifugal pump 42 is provided to pump developer composition froma supply tank 44 through the conduit 38 and into the reservoir 36. Areturn path for the developer composition is provided by another conduit46 communicating with the opposite end of the reservoir 36 and with thesupply tank 44. In this latter conduit 46 a check valve 48 is providedso that the pressure within the reservoir 36 may be adjusted to insurethat the proper amount of developer composition is fed to the developerpad 11.

In high speed recording equipment it may be desirable to develop imageson continuous webs at different speeds. Any change in development speedwill, of course, result in a change in the rate at which developercomposition is used up. In the apparatus of FIGURE 4, means are providedto vary the rate at which developer composition is supplied to reservoir36 and the developer pad 11 in accordance with the speed at which theweb 30 is drawn between the developer pad 11 and the roller 32. At oneend of the roller 32, the shaft 50 thereof is provided with a spur gear52 which in turn meshes with another spur gear 54 coupled to the pump42. Thus any change in web speed will automaticaly produce acorresponding change in pump speed to thereby vary the pressure in thereservoir 36 and the rate of feeding developer composition to thedeveloper pad 11.

What is claimed is:

1. In a method of printing including the step of producing aneletrostatic charge image on an insulating surface, the improvementcomprising developing said charge image into a visible image by firstapplying a thin film of insulating carrier liquid to said surface andthereafter contacting thereacross absorbent material having dispersedtherein a quantity of dry electroscopic developer particles.

2. A method of color electrophotography comprising the steps of applyinga thin film of insulating carrier liquid to a surface of photoconductiveinsulating material substantially uniformly charging a surface ofphotoconductive insulating material; exposing said surface to a lightimage containing all color components except one color component of anoriginal color image to produce on said surface a latent electrostaticimage representative of said one color component of said original image;developing said electrostatic image into a visible color image bycontacting across said surface absorbent material having dispersedtherein a quantity of electroscopic developer particles of said onecolor, at least that portion of said material in contact with saidsurface being wetted by an insulating carrier liquid; and repeating saidsteps at least once to produce 'an electrostatic image corresponding toa different color component of said original image and to develop saidlast mentioned electrostatic image with electroscopic developerparticles of said different color dispersed in a different absorbentmaterial.

References Cited UNITED STATES PATENTS 1,769,250 7/1930 Arvidson 118-2661,784,913 12/1930 Scott 96-1 2,463,711 3/ 1949 Nagle 204-224 2,540,6022/ 1 Thomas et a1 204-224 2,590,927 4/ 1952 Brandt et a1 204-2242,713,176 7/1955 Rosenthal (I) 15-134 2,726,200 12/ 1955 Holsapple204-224 2,750,332 6/1956 Miller 204-224 2,805,641 9/ 1957 Rosenthal (H)118-265 2,811,465 10/ 1957 Greig 96-1 2,833,702 5/ 1958 Elfers 204-2242,961,395 11/1960 Icxi 204-224 2,986,521 5/ 1961 Wielicki 96-1 3,010,88311/1961 Johnson et a1. 204-224 3,038,799 6/ 1962 Metcalfe et a1 96-13,050,768 8/1962 Rosenthal (III) 15-563 3,053,688 9/1962 Greig 96-13,081,263 3/ 1963 Metcalfe et a1 96-1 X 3,102,045 8/1963 Metcalfe et al96-1 X 3,150,976 9/ 1964 Johnson 96-1 NORMAN G. TORCHIN, PrimaryExaminer.

ALEXANDER D. RICCI, Examiner.

C. E. VAN HORN, Assistant Examiner.

1. IN A METHOD OF PRINTING INCLUDING THE STEP OF PRODUCING ANELETROSTATIC CHARGE IMAGE ON AN INSULATING SURFACE, THE IMPROVEMENTCOMPRISING DEVELOPING SAID CHARGE IMAGE INTO A VISIBLE IMAGE BY FIRSTAPPLYING A THIN FILM OF INSULATING CARRIER LIQUID TO SAID SURFACE ANDTHEREAFTER CONTACTING THEREACROSS ABSORBENT MATERIAL HAVING DISPERSEDTHEREIN A QUANTITY OF DRY ELECTROSCOPIC DEVELOPER PARTICLES.